Abstract

Coherence-domain imaging systems can be operated in a single-photon-counting mode, offering low detector noise; this in turn leads to increased sensitivity for weak light sources and weakly reflecting samples. We have demonstrated that excellent axial resolution can be obtained in a photon-counting coherence-domain imaging (CDI) system that uses light generated via spontaneous parametric downconversion (SPDC) in a chirped periodically poled stoichiometric lithium tantalate (chirped-PPSLT) structure, in conjunction with a niobium nitride superconducting single-photon detector (SSPD). The bandwidth of the light generated via SPDC, as well as the bandwidth over which the SSPD is sensitive, can extend over a wavelength region that stretches from 700 to 1500nm. This ultrabroad wavelength band offers a near-ideal combination of deep penetration and ultrahigh axial resolution for the imaging of biological tissue. The generation of SPDC light of adjustable bandwidth in the vicinity of 1064nm, via the use of chirped-PPSLT structures, had not been previously achieved. To demonstrate the usefulness of this technique, we construct images for a hierarchy of samples of increasing complexity: a mirror, a nitrocellulose membrane, and a biological sample comprising onion-skin cells.

Figures (6)

Brightness images, in which brightness is proportional to the calculated power spectral density of the emission, at various temperatures. The features of the structures are specified by the length of the first period b1 and the chirp parameter ζ. (a) Unchirped structure with b1=7.95μm and ζ=0. (b) Medium-chirped structure with b1=7.85μm and ζ=1.26×10−6μm−1. (c) Maximum-chirped structure with b1=7.5μm and ζ=6.24×10−6μm−1. (d) Calculated normalized spectrum for the maximum-chirped structure at a temperature of 80°C. The parameters were chosen to match those of the structures used in the experiments described subsequently. The bandwidth increases substantially with the chirp parameter.

(a) The generic experimental arrangement makes use of a Michelson interferometer comprising a beam splitter (BS), reference mirror, and sample. The broadband light emanating from the source is coupled into a SM fiber and collimated by lens L3. The light within the interferometer is focused onto the reference mirror and the sample using lenses L4 and L5, respectively. The lenses, reference mirror, and sample are placed on nanopositioning stages to change their positions, as indicated by the arrows. Experiments were performed using both SSPDs and SPADs as detectors. (b) The downconversion source consists of light from a cw frequency-doubled Nd3+:YVO4 laser (Coherent Verdi), operating at a wavelength of 532nm and at a power of 2W, that pumps a chirped-PPSLT device. The structure is aligned to obtain collinear SPDC. The downconverted light is collimated using lens L1 and coupled into a SM fiber via lens L2. The filter removes the pump light and allows only the downconverted light to be coupled into the fiber.

Brightness images, in which brightness is proportional to the measured power spectral density of the emission, at various temperatures, from (a) the unchirped structure, (b) the medium-chirped structure, and (c) the maximum-chirped structure. (d) Measured normalized spectrum for light from the maximum-chirped structure at a temperature of 80°C. The results bear considerable resemblance to the calculations displayed in Fig. 1.

Normalized interferograms and their envelopes versus reference-arm displacement for a mirror sample (A-scans). In all cases, the step size used in constructing the interferograms was 100nm and the duration of the counting-time windows was 300ms. (a) Downconversion/superconducting detector (SPDC/SSPD). The FWHM of the interferogram envelope was 1.6μm. The highest resolution was achieved with this combination. (b) Downconversion/avalanche detector (SPDC/SPAD). The FWHM of the interferogram envelope was 2.8μm. (c) Superluminescence/superconducting detector (SLD/SSPD). The FWHM of the interferogram envelope was 6.3μm.

Normalized interferograms versus reference-arm displacement for a pellicle sample (A-scans). In all cases, the step size used in constructing the interferograms was 100nm and the duration of the counting-time windows was 300ms. (a) Downconversion/superconducting detector (SPDC/SSPD). This combination permitted reflections from the two surfaces to be resolved. (b) Downconversion/avalanche detector (SPDC/SPAD). The two surfaces were not resolved. (c) Superluminescence/superconducting detector (SLD/SSPD). The two surfaces were not resolved.

Two-dimensional (xz) B-scans of an onion-skin sample. (a) Scan collected using broadband downconversion light and a superconducting detector (SPDC/SSPD). (b) Scan collected from the same onion-skin sample using superluminescence light and the same superconducting detector (SLD/SSPD). Higher axial resolution is attained by using downconversion, by virtue of its broader bandwidth. These cross-sectional views of the tissue highlight the relatively large reflectances at cellular surfaces, which stem from refractive-index discontinuities.